1. Field of the Invention
This invention relates generally to semiconductor optical amplifiers. More particularly, it relates to vertically lasing semiconductor optical amplifiers (VLSOAs) integrated with other optical elements on a common substrate and to methods for fabricating such integrated optical devices.
2. Description of the Related Art
Optical amplifiers, which boost the power of optical signals, are a basic building block for many types of optical systems. For example, fiber optic communications systems transmit information optically at very high speeds over optical fibers. A typical communications system includes a transmitter, an optical fiber, and a receiver. The transmitter incorporates information to be communicated into an optical signal and transmits the optical signal via the optical fiber to the receiver. The receiver recovers the original information from the received optical signal. In these systems, phenomena such as fiber losses, losses due to insertion of components in the transmission path, and splitting of the optical signal may attenuate the optical signal and degrade the corresponding signal-to-noise ratio as the optical signal propagates through the communications system. Optical amplifiers are used to compensate for these attenuations. As another example, receivers typically operate properly only within a relatively narrow range of optical signal power levels; optical amplifiers are used to boost an optical signal to the proper power range for the receiver.
Fiber amplifiers are one type of optical amplifier. They include a length of fiber which has been doped to form an active gain medium. Ions of rare-earth metals, such as erbium, are typically used as the dopant. The doped fiber is typically pumped by an optical pump at a wavelength which is preferentially absorbed by the ions but different from the wavelength of the optical signal to be amplified. The pumping results in a population inversion of electronic carriers in the active medium. Then, as the optical signal propagates through the doped fiber, it is amplified due to stimulated emission.
Semiconductor optical amplifiers (SOAs) are another type of optical amplifier. SOAs contain a semiconductor active region and an electrical current typically is used to pump the electronic population in the active region. An optical signal propagating through the active region experiences gain due to stimulated emission. Conventional SOAs are non-lasing and the gain in non-lasing SOAs typically depends on the amplitude of the optical signal. Thus, strong portions of the optical signal are amplified less than weak portions, resulting in distortion of the signal and also crosstalk between different optical signals propagating simultaneously through the amplifier. This significantly limits the use of conventional SOAs. In contrast, in lasing SOAS, the semiconductor active region is part of a laser cavity which is pumped above the lasing threshold. The gain is then clamped due to the lasing action and is fairly constant until the amplifier reaches its power limit.
Integrated optics is a term which is sometimes used to refer to the concept of an optical equivalent to electronic integrated circuits, in which many optical elements are integrated onto a common substrate. Current-day electronic circuits can implement very complex functionalities by combining a very large number of electronic elements in a single monolithic device. Electronic amplifiers are a basic building block of electronic integrated circuits. Current-day optical amplifiers, however, are not suited to play an analogous role with respect to integrated optics.
For example, fiber amplifiers are inherently too large to be useful in integrated optics. Common amplifiers used in the telecommunications industry are based on fibers which are tens of meters long. Fiber amplifiers also suffer from slow switching speeds and poor power efficiency. Furthermore, fiber amplifiers are currently manufactured using a process which is more reminiscent of electronic circuits made from hand soldered discrete components than today's highly automated semiconductor processes. As a result, not only is it difficult to produce large quantities of fiber amplifiers but the fiber amplifiers produced are fairly expensive.
SOAs also suffer from drawbacks which limit their usefulness in integrated optics. Conventional SOAs are typically packaged as discrete devices. The SOA is aligned to fiber pigtails, one for the SOA input and one for the SOA output, and the entire device is produced as a single package. This type of packaged device suffers from the same drawbacks as fiber amplifiers: large size and a labor-intensive manufacturing process.
Furthermore, in electronic integrated circuits, the various components are built up from a handful of basic building blocks: the transistor, the diode, interconnects, etc. This is not the case for optical integrated circuits. SOAs typically arc very different in design from waveguides, sources, modulators, detectors, etc. The optical elements required to build up more complex systems typically are not based on a handful of common building blocks and often even require the use of different materials systems. For example, current-day fiber communications may require the use of a multi-wavelength transmitter. Such a transmitter typically uses InP based lasers, lithium niobate modulators, a dielectric thin film wavelength division multiplexer, and a glass fiber amplifier pumped by several GaAs based lasers. Note the difference in materials and structure. Integrating these devices onto a common substrate to form an integrated transmitter is not straightforward.
Even when conventional SOAs have been successfully integrated with other optical elements, they are limited to niche applications due to their crosstalk and other performance limitations. For example, irrespective of the integration problems, conventional SOAs generally are not suited to amplifying multi-wavelength data communication signals because the crosstalk between the different wavelength signals would unacceptably degrade the signals.
Thus, there is a need for an optical amplifier which can be easily integrated with other optical elements on a common substrate. There is further a need for methods for fabricating these integrated optical devices.